I. Field of the Invention
The present invention relates to the field of devices for the generation of electromagnetic radiation of the ultraviolet and vacuum ultraviolet wavelengths useful in the manufacture and processing of semiconductor devices.
II. Discussion of the Related Art
A. Semiconductor Manufacturing Problems
The manufacture of semiconductor devices relies on the deposition of thin, even films of desired chemical composition and structure. Deposition of thin, even films requires that the surface on which the film is to be deposited to be smooth and even. Otherwise, the deposited layer will not be smooth and even. One aspect of this property of film deposition is termed "surface sensitivity." {See Kwok et al., J. Electrochem. Soc., 141(8):2172-2177 (1994); Matsuura et al., Proceedings of the 22.sup.d International Conference on Solid State Devices and Materials, Sendai, pp:239-242 (1990); Fujino et al., J. Electrochem. Soc. 138(2):550-554 (1991); and Fujino et al., J. Electrochem. Soc. 139(6):1690-1692 (1992), each incorporated herein fully by reference.} Surface sensitivity is characterized by inconsistent and variable deposition rates and increased roughness of the resulting films as the process conditions are varied. The process conditions of interest are deposition temperature, deposition pressure, mole-fraction of the reactants (e.g., TEOS and ozone), and possibly some hardware conditions specific to the design of the reactor used to deposit these films.
An approach to decrease the surface sensitivity is to modify the surface of the underlying film. Maeda et al., U.S. Pat. No. 5,387,546 taught the exposure of deposited semiconductor films to ultraviolet radiation during heating. The ultraviolet radiation was produced by a mercury lamp which generates electromagnetic radiation with wavelengths of 185 nanometers (nm) and 254 nm, as well as some longer wavelength radiation. However, because this process is carried out on layers of USG film which have already been deposited, it does not deal with the problem of surface sensitivity. Thus, there is a need for improved ways of reducing surface sensitivity.
Additionally, with increasing miniaturization of semiconductor devices, as the gaps between device features decreases, these gaps becomes increasingly more difficult to fill adequately with dielectric material. Moreover, as the surface films increase in thickness, the corresponding film does not completely fill the gap, resulting in the formation of a "void." This is especially the case if there is surface sensitivity of deposition of materials within the gap. Films which provide a conformal coating of device features, result in the formation of voids as the gaps becomes filled. These voids can trap contaminants which can degrade the integrated circuit device, and are not effective dielectrics. Thus, there is a need for improved gap filling in the manufacture of semiconductor devices.
Additionally, as circuit feature density increases, there is a need for developing new dielectric materials. Such materials include organic polymers. Deposition of these polymers can be accomplished by dissociating precursors with reactive intermediates and permitting these intermediates to polymerize on the semiconductor substrate. To improve the physical and chemical properties of these deposited materials, dissociation of precursors can be accomplished using electromagnetic radiation in the ultraviolet and vacuum ultraviolet wavelengths. However, the currently available devices for such irradiation are not well suited to optimizing the exposure of substrates and films to ultraviolet radiation. The problems and attempted solutions to these problems are discussed in more details below.
B. Ultraviolet and Vacuum Ultraviolet Lamps
Lamps for producing ultraviolet or vacuum ultraviolet radiation are known in the art, and are exemplified by the devices described in Kogelschatz, U.S. Pat. No. 5,432,398; Kogelschatz, U.S. Pat. No. 5,386,170; Eliasson et al., U.S. Pat. No. 4,837,484; Eliasson et al., U.S. Pat. No. 4,945,290; Eliasson et al., U.S. Pat. No. 4,983,881; Gellert et al., U.S. Pat. No. 5,006,758; Kogelschatz et al., U.S. Pat. No. 5,198,717; and Kogelschatz, U.S. Pat. No. 5,214,344. Each of the aforementioned references in herein incorporated fully by reference.
Dielectric barrier discharge devices consist of two conductive electrodes, at least one of which is covered with a dielectric layer and are separated from each other by an emitter moiety-containing gap. Emitter moieties are usually in the form of a gas, whose atoms, under normal temperatures and other pressures, do not form chemical bonds between them. For example, at usual temperatures and pressures, noble gases normally do not form interatomic bonds. However, under high energy conditions, such as those present in plasmas, the emitter moieties lose electrons, and therefore the moieties can form intra-atomic bonds, thereby forming "excited emitter moieties." "Excited emitter moieties" as herein defined comprise at least two emitter moieties bonded to each other under conditions of radiator operation. These bonds have high energy and are unstable, and upon their breakdown, electromagnetic radiation of wavelengths characteristic of the excited emitter moiety is emitted.
The high energy plasma used to form the excited emitter moieties is generated by dielectric discharge, which is generated when an electrical field is created between electrodes with high resistance to electric current flow between them. Electrodes are coated with a dielectric material which provides the large resistance to the flow of electrical current between the electrodes and have a high capacitance. Thus, a high voltage is required to overcome the dielectric barrier, and when the voltage is sufficiently high, the barrier is overcome, and a plasma is generated in the gas between the electrodes. Because a higher voltage is needed to initiate plasma formation between dielectric barrier electrodes, the resulting current through the gas is substantially higher than between electrodes without the dielectric layer, and consequently, more power is delivered to the gas.
C. Problems in the Art
The prior art devices, however, suffer from several problems which make them less desirable for use for the exacting needs of semiconductor manufacture. These include uneven temperature distribution within the device, uneven radiation distribution over the surface to be treated, and short useful lifetime.
1. Generation of Uneven Plasmas
The first problem is that for each plasma microfilament generated between electrodes, the dielectric barrier is overcome at only a few sites along the electrode's surface. Once a site of plasma microfilament formation has been established, the dielectric material at that site can be degraded, thereby decreasing its resistance to electrical current. As the resistance decreases, that site becomes the focus for subsequent discharges. Repeated discharges further exacerbate the dielectric degradation and results in locally high temperatures within the dielectric material. This leads to further degradation of the properties of the dielectric. As the dielectric material becomes degraded, it loses the ability to store charge, the threshold voltage for dielectric discharge decreases, resulting in progressive loss of electrical power and weakening of the plasma field. Because the power output of the excimer device is related to the power of the electrical discharge, loss of electric power results in decreased radiation output from the device. Thus, the useful lifetime of the excimer device is limited.
2. Temperature Regulation
Another problem is that during use, the electrodes and emitter gas can be overheated. Overheating the electrodes alters the wavelength of the emitted radiation and can lead to degradation of the electrode dielectric material, reducing the useful lifetime of the lamp. Although the exact mechanism of this overheating is not known, it is known that plasma microfilaments are not evenly distributed in the discharge spaces of the prior art devices. This uneven distribution of plasma results in variations of gas temperature in different parts of the devices.
As a result of temperature differences within the discharge tube, there are differences in the wavelengths of the emitted radiation generated by the lamp. This results in a broader spectrum of emitted radiation, which can have undesired consequences on semiconductor processing. When working with precursor materials for film deposition, the energy required to dissociate a particular type of chemical bond is narrow. Controlled processing of these precursor molecules requires the precise control of the energy of the radiation used to dissociate the desired bonds.
However, broadening the spectrum of radiation can break bonds within the semiconductor material which are not intended to be broken, leading to a heterogeneous mixture of precursor molecules, some of which are not desired and actually contaminates the film to be deposited. Furthermore, exposing a surface to broadened spectrum of radiation can lead to degradation of the semiconductor surface or formation of undesired species of radicals. For example, in the case of Xe, when the temperature of the excimer gas exceeds about 300.degree. C., the interatomic bonds between xenon atoms do not form. Thus, without interatomic bonds to breakdown, there is no excimer radiation emitted by the device.
Thus, a significant problem in the art is how to overcome these differences in temperatures locally within the devices, and how to keep the emitters cool enough to efficiently generate UV or VUV radiation. Prior art devices have incorporated water to both cool the devices and provide more even temperatures. However, because water has a very high dielectric constant (K=81), water conducts electrical fields easily, thereby providing an alternative pathway for electrical energy to flow away from the excimer gas. This leakage of current to electrical ground requires a much higher power output from the AC source to generate the needed electrical power to the gas to initiate and maintain lamp operation.
3. Uneven Distribution of Radiation
Another problem specific to the manufacture of semiconductor devices is the desirability of providing even distribution of the radiation onto the surface of a semiconductor wafer. Several of the prior devices consist of tubes in which a central electrode is surrounded by an outer electrode. Examples of such devices are found in Kogelschatz, U.S. Pat. No. 5,013,959, Kogelschatz, U.S. Pat. No. 5,386,170, Kogelschatz, U.S. Pat. No. 5,432,398, and Kogelschatz et al., U.S. Pat. No. 5,198,717. Each of these patents is herein incorporated fully by reference. Because the discharge tubes of these devices emit radiation radially in all directions, they do not provide even emission of radiation in a particular desired direction. This is because the intensity of the radiation is altered by its passing through the elements of the device, such as quartz tubes, electrodes, and by passing through the emitter gas itself. The electromagnetic power not reaching the wafer is therefore wasted. Additionally, the geometry of cylindrical emitters with a central electrode surrounded by the electrode inherently produces uneven radiation. Uneven distribution of radiation results in uneven exposure of the flat wafer, thereby resulting in uneven treatment of the surface, thereby reducing the precision and consistency of the exposure of a semiconductor wafer to the radiation.
Attempts in the prior art to overcome uneven distribution include devices such as those exemplified by Gellert et al., U.S. Pat. No. 5,006,758, Eliasson et al., U.S. Pat. No. 4,983,881, Eliasson et al., U.S. Pat. No. 5,173,638, and Eliasson et al., U.S. Pat. No. 4,945,290. Each of the above patents is herein incorporated fully be reference. These devices suffer from the same problem of localized loss of dielectric efficacy and uneven plasma generation. Additionally, with many of the prior art devices, there is no means for cooling the electrodes, and further degradation of the dielectric material and heating of the excimer gas results in inefficiency and short useful device lifetime.
4. Contamination of Radiators
An additional problem in the art is that with use, excimer lamps become contaminated with by-products of the impurities within the excimer gas, and from the dielectric material. These contaminants deposit on the window of the lamp, thereby decreasing its transparency to the UV or VUV radiation. This contamination reduces the useful lifetime of the device.
Thus, there are persistent problems in the art to design and manufacture excimer devices which remain cool during use, have long useful lifetimes, have narrow spectra in the desired wavelength ranges, have even distribution of intensity of the emitted radiation and are convenient and relatively inexpensive to manufacture.